Process for making metal oxide nanoparticles

ABSTRACT

There is described a process for preparing metal oxide particles which are substantially free of coarse tail from an oxidizing agent and a vaporous metal reactant in a flow reactor; comprising, (a) directing a flow of the metal reactant into a contacting region of the flow reactor; comprising (a) passing a flow of oxidizing agent through a high temperature zone of the flow reactor to form a flow of hot oxidizing agent and directing the flow of the hot oxidizing agent onto the contacting region of the flow reactor at a flow condition sufficient to form a reaction stream comprising a flow of hot oxidizing agent, a flow of metal reactant and a diffusive flow of the hot oxidizing agent and the metal reactant, the temperature of the hot oxidizing agent being at least sufficient to initiate oxidation of the metal reactant in the diffusive flow; (c) passing the reaction stream into a reaction zone of the flow reactor, while simultaneously introducing a flow of an upper cooling fluid substantially coaxially with the reaction stream to form a fluid curtain which surrounds the reaction stream; (d) maintaining the fluid curtain while the metal oxide particles form within the diffusive flow of the reaction stream and until at least a major portion of the reaction stream is cooled to a temperature below the temperature at which metal oxide particles coalesce; and (e) separating the metal oxide particles from the reaction stream.

CROSS-REFERENCE TO RELATED APPLICATIONS

Cross-reference is made to U.S. provisional application No. 60/589,232filed on Jul. 20, 2004, which is incorporated herein by reference of itsentirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for producing nanoparticles and moreparticularly to a process for reacting a metal reactant and an oxidizingagent to make nanoparticles, the process being capable of eliminatingcoarse tail and scale formation.

2. Description of the Related Art

The chloride process for making titanium dioxide includeshigh-temperature anhydrous vapor phase reactions where liquid titaniumtetrachloride is vaporized and superheated then reacted with oxygen toproduce titanium dioxide. The superheating and subsequent reaction phasecan be carried out either by a refractory process, where the reactantsare heated by refractory heat exchangers and combined. Alternatively,carbon monoxide can be purified and then mixed with the titaniumtetrachloride and oxidizing agent and then the mixture subjected to acontrolled combustion. Another method is by vaporizing the titaniumtetrachloride in a hot plasma along with the oxidizing agent.

The development of these processes for the production of fine particleswhich are below about 100 nm in size, termed “nanoparticles”, has been apoint of focus in recent years. In particular, titanium dioxidenanoparticles have gained increased attention because they can have ahigh degree of transparency and they can also have UV protectiveproperties. The combined properties of transparency and UV protection isespecially desirable in applications demanding both propertiesincluding, without limit, cosmetics; product coatings, such asautomotive clear coatings and wood coatings; and plastics, such aspolymer composites.

The development of processes for making nanoparticles continues to be achallenge.

The build-up of scale within the reactor is a significant problem in theproduction of metal oxide nanoparticles, particularly titanium dioxidenanoparticles. Scale is a layer of solids formed on the walls of thereactor that can build up significantly overtime as the hot metal oxideparticles and reactants collide with the walls of the reactor and stickat a temperature at which the metal oxide particles can coalesce. Thelayer can comprise sintered metal oxides which are very hard andtenacious. This hard and tenacious type of reactor wall scale is laborintensive to remove and represents loss of product which increasesproduction costs.

In the production of titanium dioxide nanoparticles the presence of“coarse tail” can be a significant problem. “Coarse tail” is an amountof large particles, typically having a diameter exceeding about 100 nmand greater, present in the product. The large particles can be built upfrom smaller metal oxide particles and/or reactants which collide witheach other and coalesce at a high temperatures. In addition, the largeparticles can result from particle aggregates that can form frompartially coalesced particles. Further, a “soft” layer of largecoalesced particles that can form on the walls of the reactor can becomeentrained with the flow of product and contribute to coarse tail.

In the manufacture of titanium dioxide nanoparticles, coarse tail can bea commercialization barrier because it is considered detrimental totransparency. Even a very small percentage of titanium dioxide particleshaving a diameter above about 100 nm can impart a degree of opacitysufficient to render the product unacceptable for high transparencyapplications such as automotive clear coatings. Since, large particlescan be difficult and costly to remove there is a need for processescapable of producing nanoparticles which are free of coarse tail.

In U.S. Pat. No. 6,277,354 at Col. 4, lines 37-41 this “stickiness”property of metal chlorides and metal oxides which can lead to wallscale and coarse tail is defined as meaning that the ratio of thetemperature Kelvin of the particular particles to their melting pointtemperature Kelvin is equal to or less than about ⅔.

A highly turbulent quench zone has been described for controllingparticle size distribution and reactivity to overcome particle growthand agglomeration. Highly turbulent quenching conditions can alsoprovide high conversion rates. While relatively high conversions ofreactants can be an advantage of this process, coarse tail and reactorwall scale problems remain. Highly turbulent conditions promotecollisions between particles which at high temperatures increaseparticle coalescence which increases the proportion of large particlesand the buildup of reactor wall scale.

SUMMARY OF THE INVENTION

The present invention has been found to achieve particle size controlwithout substantial formation of wall scale in a process forsynthesizing metal oxide nanoparticles by reaction of metal reactant andan oxidizing agent.

Even when a layer of “dust” does form on the walls during the process ofthis invention, the dust does not sufficiently accumulate to require thereactor to be shut down for cleaning. Further, such dust does notcontribute in any substantial way to product coarse tail.

The present invention can significantly reduce and even eliminate coarsetail, that is metal oxide particles which exceed about 100 nm indiameter. Thus, when titanium dioxide is made by the process of thisinvention the titanium dioxide is especially useful for applicationsrequiring a high degree of transparency.

The invention is directed to a process for preparing metal oxideparticles which are substantially free of coarse tail from an oxidizingagent and a metal reactant in a flow reactor, comprising:

(a) directing a flow of the metal reactant into a contacting region ofthe flow reactor;

(b) passing a flow of oxidizing agent through a high temperature zone ofthe flow reactor to form a flow of hot oxidizing agent and directing theflow of the hot oxidizing agent onto the contacting region of the flowreactor at a flow condition sufficient to form a reaction streamcomprising a flow of hot oxidizing agent, a flow of metal reactant and adiffusive flow of the hot oxidizing agent and the metal reactant, thetemperature of the hot oxidizing agent being at least sufficient toinitiate oxidation of the metal reactant in the diffusive flow;

(c) passing the reaction stream into a reaction zone of the flowreactor, while simultaneously introducing a an upper cooling fluidsubstantially coaxially with the reaction stream to form a fluid curtainwhich surrounds the reaction stream;

(d) maintaining the fluid curtain while the metal oxide particles formwithin the diffusive flow of the reaction stream and until at least amajor portion of the reaction stream is cooled to a temperature belowthe temperature at which metal oxide particles coalesce; and

(e) separating the metal oxide particles from the reaction stream.

In another embodiment, the invention is directed to a process forpreparing metal oxide particles which are substantially free of coarsetail from an oxidizing agent and a metal reactant in a flow reactorhaving in order a high temperature zone, an intermediate zone, acontacting region and a reaction zone, comprising:

(a) directing a flow of the metal reactant into a central portion of thecontacting region;

(b) passing a flow of oxidizing agent through the high temperature zoneof the flow reactor to form a flow of hot oxidizing agent and directingthe flow of the hot oxidizing agent into the intermediate zone forestablishing a laminar or near laminar flow of the hot oxidizing agentand passing the laminar or near laminar flow of hot oxidizing agent intothe contacting region of the flow reactor to form a reaction streamcomprising a diffusive flow of hot oxidizing agent and the metalreactant, the temperature of the hot oxidizing agent being at leastsufficient to initiate oxidation of the metal reactant in the diffusiveflow;

(c) passing the reaction stream into a reaction zone of the flowreactor, while simultaneously introducing a laminar or near laminar flowof an upper cooling fluid substantially coaxially with the reactionstream to form a fluid curtain which surrounds the reaction stream;

(d) maintaining the fluid curtain while the metal oxide particles formwithin the diffusive flow of the reaction stream and until at least amajor portion of the reaction stream is cooled to a temperature belowthe temperature at which metal oxide particles coalesce and when themetal oxide conversion is no greater than 90%; and

(e) separating metal oxide particles from the reaction stream, less than10% by weight of the metal oxide particles being greater than about 100nm in diameter.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments of the invention are described below with referenceto the following drawings.

FIG. 1 is a simplified schematic diagram of a flow reactor of theinvention.

FIG. 2A is a simplified schematic cross-sectional view taken along line2A-2A of FIG. 1.

FIG. 2B is a simplified schematic cross-sectional view taken along line2B-2B of FIG. 1.

FIG. 2C is a simplified schematic cross-sectional view taken along line2C-2C of FIG. 1.

FIG. 2D is a simplified schematic cross-sectional view taken along line2D-2D of FIG. 1.

FIG. 2E is a simplified schematic cross-sectional view taken along line2E-2E of FIG. 1.

FIG. 2F is a simplified schematic cross-sectional view taken along line2F-2F of FIG. 1.

FIG. 2G is a simplified schematic cross-sectional view taken along line2G-2G of FIG. 1.

FIG. 2H is a simplified schematic cross-sectional view taken along line2H-2H of FIG. 1.

FIG. 2I is a simplified schematic cross-sectional view taken along line2I-2I of FIG. 1.

FIG. 3 is a simplified schematic diagram of a conduit of the invention.

FIG. 4 is a simplified schematic diagram of an alternative conduit ofthe invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to a flow reactor and a process that promotesreaction conditions suitable for avoiding and possibly eliminating theformation of wall scale and coarse tail in the product.

The reactants are contacted in a manner that avoids rapid formation of awell-mixed reaction stream and avoids rapid quenching of the reactionstream. In the present process, the reaction stream is at leastinitially nonhomogeneous and is gradually cooled within the reactionzone as the metal oxide reaction product forms. While the conversion ofmetal reactants of the instant invention can be incomplete, typically nogreater than about 90%, more typically less than about 85% and stillmore typically less than about 80% and sometimes as low as about 50 toabout 70%, the reaction product coarse tail can be less than about 10%by weight of particles exceeding about 100 nm in diameter, moretypically less than about 5% by weight particles exceeding about 100 nmin diameter, still more typically less than about 2% by weight particlesexceeding about 100 nm in diameter. Reaction product coarse tail, thatis particles exceeding about 100 nm in diameter, can even be eliminated.Coarse tail can be determined by techniques well known in the art ofnanoparticle synthesis such as dynamic light scattering.

Because it is easier to separate unreacted material from the reactionproduct than it is to separate the coarse tail from the reaction productthe incomplete conversion of metal reactants of the instant inventiondoes not pose significant production problems.

Additionally, the inner walls of the reaction zone are buffered by afluid curtain surrounding the reaction stream to prevent the reactionstream or at least the components of it from contacting the reactorwalls at temperatures at which the metal oxide particles can coalesceand form wall scale.

The flow reactor of this invention and process of operation describedherein utilize high temperature heating that can be provided in a plasmato produce metal oxide nanoparticles. The high temperature heatingcontemplated provides local temperatures ranging from about 5,000° C. toabout 20,000° C. in the plasma gas and from about 500° C. to about 2000°C. in the reaction zone.

Metal oxide nanoparticles are synthesized by bringing an oxidizing agentto an elevated temperature, contacting the hot oxidizing agent with ametal reactant which is at a temperature below the temperature of theoxidizing agent, the hot oxidizing agent providing sufficient heat forreaction to produce a metal oxide.

The flow of the reaction stream comprising hot oxidizing agent and metalreactant has a velocity sufficient for the reaction stream to flowdownstream through a reactant contacting region and into a reaction zoneat subsonic velocity.

As the reaction stream and cooling fluid flow downstream the reactionstream is cooled as the cooling fluid intermingles with the reactionstream substantially by diffusion until the temperature of the reactionstream is below reaction temperature. This occurs prior to completemetal oxide conversion. Typically, the reaction stream is cooled tobelow a temperature suitable for oxidation when the metal oxideconversion is at least about 40%, more typically at least about 50% andeven more typically at least about 60%.

The rate of cooling is gradual and can be optimized to balance thecoarse tail formation against the completeness of the metal oxideconversion. Ideally the percent conversion is sufficient to providemetal oxide nanoparticles product, as withdrawn from the reactor,containing less than about 10% by weight particles exceeding about 100nm in diameter, more typically less than about 5% by weight particlesexceeding about 100 nm in diameter and still more typically less thanabout 2% by weight particles exceeding 100 nm in diameter.

Referring to the drawings, in FIG. 1 there is a flow reactor 5vertically disposed and having a generally tubular configuration. A flowreactor can provide continuous production. The high temperature zonetypically employs an induction plasma jet assembly. A reaction zone 16is located downstream of the high temperature zone 12. An intermediatezone 10 is located between the high temperature zone and the reactionzone 16. The intermediate zone terminates in a contacting region 14which can be substantially funnel-shaped as shown in FIG. 1.

The reactant contacting region 14 receives the flow of metal oxidereactant and provides a region for combining the flows of the metaloxide reactant and hot oxidizing agent heated in the high temperaturezone 12 to form a reaction stream. Typically the reaction streamcomprises a flow of hot oxidizing agent, a flow of metal reactant and adiffusive flow of the hot oxidizing agent and the metal reactant. Moretypically the reaction stream comprises the flow of hot oxidizing agentabout the flow of metal reactant and a diffusive flow of hot oxidizingagent and the metal reactant located therebetween. More particularly,the diffusive flow is located at a boundary region between the flows ofhot oxidizing agent and metal reactant. In the diffusive flow, themolecules of oxidizing agent and metal reactant interminglesubstantially because of their spontaneous movement caused by thermalagitation. While flow velocity can also contribute to the interminglingof the reactants, the conditions within the contacting regionestablished by the flows of the hot oxidizing agent and the metalreactant avoid rapid mixing of the reactants. Typically, the conditionswithin the contacting region comprise laminar or near laminarconditions. A region of high turbulence is created within the reactantcontacting region when the metal reactant is directed as a jet into thecontacting region but that high turbulence region is surrounded by thelaminar or near laminar flow of oxidizing agent. Under such conditionsthe reactants are able to contact each other substantially by diffusionto form the metal oxide reaction product.

A plasma jet assembly can be utilized as the heating means of the hightemperature zone 12. A plasma is a high temperature luminous gas whichis at least partially ionized. The plasma used to heat the oxidizingagent is produced by the plasma jet assembly by passing a gas, referredto as a plasma-forming gas, through a high frequency electromagneticfield, such as a radio frequency field. This electromagnetic fieldshould have a power level sufficiently high to cause, by induction, thegas to ionize and thereby produce and sustain the plasma. Theplasma-forming gas could be any gas which will ionize when subject tothe high frequency electromagnetic field and which remains inert when inthe presence of a reactant. Examples of suitable plasma-forming gasesinclude helium, argon, carbon monoxide, oxygen, air or a mixturethereof. By supplying a high frequency electric current to the inductivecoil 52 the gas in the high temperature zone 12 is ionized and theplasma is created.

When plasma is used for the high temperature zone the plasma onceestablished may be sustained solely by the flow of oxidizing agent intothe high temperature zone 12. In addition, the plasma can be initiatedand established by the flow of oxidizing agent. Typically an inertreadily ionized plasma-forming gas such as argon initiates the plasmainto which the oxidizing agent is introduced.

Plasma generating torches useful in this invention are well known tothose skilled in this field.

Referring to FIG. 1 the plasma-forming gas is introduced into the flowreactor 5 via a plasma-forming gas inlet 17 to initiate and establishthe plasma. The oxidizing agent is introduced to the flow reactor viainlet 18. The inlet for the oxidizing agent and flow of oxidizing agentis shown in FIG. 2C. As the oxidizing agent surrounds the plasma andmixes with the plasma it is heated to a very high temperature. Typicallythe temperature to which the oxidizing agent is elevated ranges fromabout 500 to about 2000° C. It will be apparent to those skilled in thefield that the temperature to which the oxidizing agent is heated mayvary depending upon the choice of oxidizing agent, other reactants andthe desired reaction product. The oxidizing agent flows downstreamthrough a sleeve 19, formed about at least a portion of a metal reactantfeed conduit 22, which directs the flow of oxidizing agent through thehigh temperature zone 12 and into the reactant contacting region 14. Inone embodiment the oxygen flows downwards directed by a sleeve separator15 which divides an upper portion of the sleeve into two regions abovethe high temperature zone which serves to keep the flow of the oxidizingagent separate from the flow of the plasma-forming gas. The use of thesleeve separator 15 has been found to be beneficial for establishing theflow of oxidizing agent suitable for the process, keeping the plasma gaswithin the hottest part of the high temperature zone, providingoxidizing agent sheathing on the walls of the reactor and keeping thetemperatures of the walls low. The sleeve separator 15 is also shown inFIG. 2C.

The metal reactant feed conduit 22 is positioned along the vertical axisof the flow reactor 5 passing through the high temperature zone 12 andterminating in the reactant contacting region 14. Because of theconfiguration of the plasma, the temperature along the central axisbounded by the RF induction coil is cooler relative to regions radiallydistant from the central axis. The metal reactant feed conduit can bepassed through this cooler region of the plasma. Temperatures in thislow temperature zone can be readily calculated by those skilled in theart of plasma technology. The metal reactant, typically introduced via ametal reactant feed inlet 23 located in the center of and at the top ofthe reactor, flows through the metal reactant feed conduit to thereactant contacting region 14. The outlet for the metal reactant feed islocated upstream from and adjacent to the reactant contacting region.

In one embodiment of the invention the metal reactant feed conduit 22comprises concentric tubes having an inner tube 34 having a tip 35 andan outer tube 36 having a tip 37; the inner tube for conveying the metalreactant optionally together with a carrier gas, such as argon, into thereactant contacting region and the outer tube for conveying asupplemental fluid into the reactant contacting region. The supplementalfluid can be a reactant or inert fluid, a coolant, adjuvant or dopant.Examples of suitable supplemental fluids include water or co-metalreactants including without limitation aluminum, silicon, cesium,manganese, vanadium or tin. FIG. 2A shows the inlet of the outer tube 36for conveying the supplemental fluid and the inner tube 34.

In one embodiment the tip of the outer tube 37 is recessed from the tipof the inner tube 35 which is especially useful when a fluid is passedthrough the outer tube and flows about the tip of the inner tubesufficient to prevent scale from forming on the tip of the conduit and,additionally, into the reactant contacting region and, additionally,into the reaction zone. With this configuration the tip of the innertube can be maintained substantially free of scale, preferablycompletely free of scale. A typical fluid is oxygen or argon gas.

In one embodiment, to increase throughput, the metal reactant feedconduit is configured so that the metal reactant is conveyed through anannulus about a central channel. As shown in FIG. 3, the metal reactantfeed conduit can also comprises a central channel 100 through which afluid such as oxygen can flow, a metal reactant annulus 102 throughwhich the metal reactant can flow, an inert gas annulus 104 throughwhich an inert fluid such as argon can flow. Optionally the metalreactant feed conduit can have a water jacket about its perimeter forcooling. As shown in FIG. 4, to minimize heat loss while permittingincreased throughput an expanded region 108 of the metal reactant feedconduit can be located below the high temperature zone.

The metal reactant feed can be introduced into the flow reactor inliquid or vapor form. Typically, the metal reactant is vaporized priorto being introduced to the flow reactor. The metal reactant can bevaporized by any of various techniques well known in the metalvaporization art.

The intermediate zone 10 allows the oxidizing agent to heat by mixingwith the plasma-forming gas (recognizing that some amount of mixing ofthe plasma-forming gas and the oxidizing agent can occur in the hightemperature zone). The intermediate zone can also establish a laminar ornear laminar flow profile of the oxidizing agent and coaxial flow withthe metal reactant. In a lower portion of the intermediate zone thegases cool sufficiently to no longer be in the plasma state.

When the metal reactant flows from the outlet of the metal reactant feedconduit 24 it meets the flow of hot oxidizing agent and together theyform a reaction stream which flows generally in a direction downstreamtowards the reaction zone. While the metal reactant flowing from theconduit can be a jet resulting in a region of high turbulence, themixing of the reactants is substantially by diffusion, not turbulence,since the flows meet substantially parallel to each other. Certainconfigurations of the contacting region, for example without limitation,when a funnel-shaped contacting region is employed, may cause at least aportion of the flows to meet at a slight angle depending upon the flowvelocity of the hot oxidizing agent and/or the shape of the contactingregion and thus the flows may not meet completely in parallel to eachother. However, such conditions in the contacting region facilitatemixing substantially by dispersion without inducing highly turbulentconditions which would promote rapid mixing which can promote coarsetail and scale formation. A configuration that promotes a highlyturbulent condition and rapid mixing within the contacting region suchas a jet of oxidizing agent introduced substantially perpendicular tothe jet of metal reactant is avoided.

A funnel-shaped reactant contacting region 14 can be utilized to directthe flow of oxidizing agent towards the flow of the metal reactant andinto the reaction zone 16.

In the reactant contacting region 14 and even in the upper region ofreaction zone, the nonhomogeneous reactant stream can be characterizedby a high concentration of metal reactant located along the verticalaxis of the reactant contacting region, a boundary region of a diffusiveflow in which the molecules of oxidizing agent and metal reactantintermingle, a high concentration of hot oxidizing agent surrounding theboundary region and at the outermost region of the reaction stream.

The high concentration of hot oxidizing agent at the outermost region ofthe reaction stream can form a fluid curtain surrounding the reactionstream which can prevent coalesceable material in the reaction streamfrom contacting the walls of the reactant contacting region which keepsthe walls substantially free of wall scale, preferably completely freeof wall scale. More particularly, the fluid curtain of hot oxidizingagent can buffer the inner walls of the reactant contacting region.

The temperature of the metal reactant is lower than the temperature ofthe oxidizing agent. Typically the temperature of the metal reactant asit emerges from the outlet ranges from about 100 to about 200° C., thehot oxidizing agent providing sufficient heat for reaction.

Typically the flow velocities within the flow reactor are subsonic. Thelocal flow velocities of the oxidizing agent in the high temperaturezone range from about 100 to about 400 ft/s. The local flow velocitiesof the metal reactant as it emerges into the reactant contacting regionrange from about 100 ft/s to about 600 ft/s. The local flow velocitiesof the reaction stream within a lower portion of the reaction zone rangefrom about 100 to about 2000 ft/s.

The metal oxide reaction product can start to form in the reactantcontacting region 14. However, the conditions of the process are suchthat the highest metal oxide conversion occurs in the reaction zone 16,downstream from where the reactant flows initially meet, and typicallyat about the middle of the reaction zone.

FIGS. 2H and 2I are greatly simplified for ease of understanding theflows within the reaction zone and do not limit the scope of theinvention. Referring to FIG. 2H, a high concentration of metal reactant60 is clustered about the vertical axis of the reactor. The stream ofmetal reactant is surrounded by a diffusive flow 62 comprising the hotoxidizing agent, metal reactant and metal oxide product which issurrounded by a flow of hot oxidizing agent 65 which is, typically,substantially free of metal reactant and more typically free of metalreactant. The flow of upper cooling fluid 66 surrounds the flow of hotoxidizing agent and is bounded by the baffle 44. The space for the lowercooling fluid 70 is bounded on a first side by the baffle 44 and on asecond side by the water jacket 41 shown in FIGS. 1, 2E, 2F and 2G. Incontrast, in the lower region of the reaction zone, shown in FIG. 2I,because of the increased opportunities for the hot oxidizing agent andthe metal reactant to intermingle with each other through diffusion, therate of which increases as the reaction stream flows downstream, theconcentration of metal reactant is reduced and the concentration ofmetal oxide reaction product is increased. Thus, as shown in FIG. 2I,the reaction stream comprises the diffusive flow 62 surrounded by theupper cooling fluid 66 which is bounded by the baffle 44. The space forthe lower cooling fluid 70 is still bounded on a first side by thebaffle 44 and on a second side by the water jacket 41 shown in FIG. 1.Conversion to metal oxide product can be highest in this lower region ofthe reaction zone. However, metal oxide conversion slows as the reactionstream comes into contact with the lower cooling fluid passing into thereaction zone via the lower cooling fluid outlet 38.

Thus, a significant amount of metal oxide conversion occurs in thereaction zone 16, downstream from where the reactant flows initiallymeet, and after an upper cooling fluid is introduced to the reactionzone.

The temperature across the reaction zone is not constant. The reactionzone contains a relatively cool upper cooling fluid as well as anonhomogeneus reaction stream, the streams flowing substantiallyparallel to each other. Not only do the reaction stream and uppercooling fluid flow as substantially discrete streams but there arelocalized areas comprising various mixtures of the components(comprising upper cooling fluid, hot oxidizing agent, metal reactant andmetal oxide reaction product) and localized areas of unmixed componentsalso flowing as substantially discrete streams having differenttemperatures. The average of the temperatures within the reaction zonecan cover a wide range, typically from about 200° C. to about 2000° C.It has also been found through computer modeling that the flow rates arenot uniform across the reaction zone. Working from the top of thereaction zone to the bottom, the highest flow rates occur at the outletof the funnel and the lowest flow rates are at the bottom of thereaction zone. Working from the inner wall of the reaction zone towardsthe center the lowest flow rate is closest to the inner wall and thehighest is along the vertical axis of the reaction zone.

In one embodiment, the flow reactor comprises a typical plasma jetassembly to which an extender pipe 40 is mounted. The extender pipe 40provides a jacket 41, typically a water jacket, and a nozzle throughwhich the reactant stream and upper cooling fluid pass into the reactionzone 16. The water jacket has a first segment 42 and a second segment 43and a baffle 44. The water jacket provides continuous or semicontinuousflow of cool water about a lower portion of the flow reactor, forcooling. The cooling water is introduced to the first segment via inlet46 at a flow rate such that it flows upwards towards the plasma, and isdirected down into the baffle by a diverter 45. The water then flows outof the baffle and is directed by the diverter 45 into a first segmentflowing towards the plasma and carrying heat away from the reaction zone16. Any suitable coolant may be used. Water or another liquid coolant isespecially useful because it will conform to the flow path design of thewater jacket. Cooling may also be effectively provided by water coolingcoils.

In a typical embodiment, a cooling fluid, typically a gas, such asoxygen gas is introduced to the reaction zone to provide a flow ofcooling fluid simultaneously with and in a direction substantiallycoaxial to the reaction stream for cooling under low turbulence. Thecooling fluid can be introduced as a laminar or near laminar flow.Typically, a plurality of cooling fluid inlets are positioned along thelength of the reaction zone to introduce cooling fluid to the reactionstream as it travels downstream.

The Reynolds Number of a near laminar flow is typically is in the rangeof about 2000 to about 4000. The Reynolds Number of a laminar flow wouldbe less than 2000. For the sake of comparison, highly turbulent flowwould be greater than about 5000.

Referring to FIG. 1 there is shown where the cooling fluids can beintroduced in two locations of the extender pipe 40. An upper coolingfluid inlet 32 is above the outlet of the metal reactant feed conduit 24and just below the high temperature zone 12. FIGS. 2E and 2F show anupper cooling fluid inlet 32. A perforated ring 33 through which theupper cooling fluid can flow is shown in FIG. 2F. A perforated ring canbe used about any region where one or more of the cooling fluids areintroduced to the reactor. A perforated ring is best shown in FIG. 2F.The perforated ring can be used to facilitate an even flow distributionof cooling fluid into the reactor.

The upper cooling fluid flows downwards between the first segment 42 ofthe water jacket 41 and the outer wall of the reactant contacting region14 and then is discharged via outlet 30 to flow co-current with thereaction stream into the reaction zone. The temperature of the uppercooling fluid is typically around room temperature.

The lower cooling fluid inlet 39 permits a lower cooling fluid to flowdownwards between the second segment 43 of the water jacket 41 and thebaffle 44. Below the baffle 44 the cooling fluid is discharged so thatit flows co-current together with the reaction stream through at least aportion of the reaction zone. The temperature of the lower cooling fluidis typically around room temperature.

The cooling fluid more particularly serves to keep the components of thereaction stream away from the walls of the reaction zone and to keep thewalls of the reaction zone at a temperature below the temperature atwhich the components of the reactant stream will stick to the walls ifthey do contact the walls. This keeps the walls substantially free ofwall scale, preferably completely free of wall scale. Thus the coolingfluid can form a fluid curtain surrounding the reaction stream to bufferthe inner wall of the reaction zone.

The flow reactor typically operates continuously. The metal oxideparticles formed in the reaction zone can be captured by any suitabletechnique such as by way of a filter. Gas-phase products of reaction andreactants can be scrubbed and treated by any suitable technique.Scrubbing and treating techniques are well known in the art.

The nanoparticles produced by this invention generally containsparticles (agglomerates) less than 100 nm in diameter, which can providemany different useful properties (electronic, optical, electrical,magnetic, chemical, and mechanical), making them suitable for a widevariety of industrial applications. The surface area of thenanoparticles can range from about 20 m²/g to about 200 m²/g. Theparticle size uniformity is suitable for nanoparticles applications.Typically, the particle size distribution d₁₀ can range from about 10 toabout 30, d₅₀ from about 30 to about 60 and d₉₀ from about 60 to about90.

It is contemplated that this invention will be suitable for productionof a wide variety of metal oxide nanoparticles in addition to titaniumdioxide, including, without limit, SiO₂, ZrO₂, ZnO, CeO₂, and Al₂O₃. Anymetal reactants capable of forming the desired metal oxides may beemployed including, without limit, a metal halide such as titaniumtetrachloride, or an organo metallic such as an organo titanate.Examples of oxidizing agents include, without limit, oxygen, air, orwater.

Working downwards from the top of the flow reactor shown in FIG. 1 thepoint of introduction of the various materials into the flow reactor ofone embodiment of the invention is shown. The metal reactant, which canbe titanium tetrachloride, is fed into the reactor through an inlet 23positioned at the center of the reactor that is shown in FIG. 1. FIG. 2Ashows introduction of argon gas about the metal reactant conduit. At thepoint of introduction, the flow rate of the metal reactant can rangefrom about 10 to at least about 240 grams per minute (gram/m), as shownin the example the flow rate was about 10 gram/m and the temperature canrange from about 20 to at least about 200° C. Argon gas is fed into theouter annular conduit 36 that is shown in FIGS. 1 and 2A. At the pointof introduction, the flow rate of the argon gas can range from about 10sl/m to about 100 sl/m as shown in the example the flow rate was about27 sl/m and the temperature can be at about room temperature. Theplasma-forming gas, which is typically argon is fed into the reactorfrom a plasma-forming gas inlet 17 that is shown in FIGS. 1 and 2B. Atthe point of introduction, as shown in the example, the flow rate of theplasma-forming gas was about 30 sl/m and the temperature can be at aboutroom temperature. The oxidizing agent which is typically oxygen but canbe air is fed into the reactor through an oxidizing agent inlet 18 thatis shown in FIGS. 1 and 2C. At the point of introduction, the flow rateof oxygen, as shown in the example was about 300 sl/m and thetemperature can be at about room temperature. The upper cooling fluidwhich is typically oxygen but can be air is fed into the reactor throughan upper cooling fluid inlet 32 as shown in FIGS. 1 and 2E. FIG. 2Gshows the space for the upper cooling fluid inside the water jacket 41.FIG. 2H, simplified for exemplification, shows the upper cooling fluid66 flowing as a distinct stream between the baffle 44 and the diffusiveflow 62. At the point of introduction, the flow rate of the uppercooling fluid can range from about 100 sl/m to about 800 sl/m as shownin the example the flow rate was 440 sl/m and the temperature can be atabout room temperature. The lower cooling fluid which is typicallyoxygen but can be air is fed into the reactor through side inlet 39 asshown in FIG. 1. The lower cooling fluid is passed into the flow reactorvia outlet 38. FIG. 2I shows the space for the lower cooling fluid 70located between the baffle 44 and the water jacket 41. At the point ofintroduction, the flow rate of the lower cooling fluid can range fromabout 100 sl/m to about 800 sl/m as shown in the example the flow ratewas 440 sl/m and the temperature can be at about room temperature. Thecooling water is fed into the water jacket of the flow reactor throughcooling water inlet 46 shown in FIG. 1.

The flow reactor can be made of stainless steel, which is usually watercooled or a corrosion resistant material such as an Ni/Fe alloy. Themetal reactant feed conduit 22 can be made from a corrosion resistantmaterial capable of withstanding high temperature oxidizing conditionsand the presence of chlorine, such as an nickel-chromium-iron alloy(inconel alloy) which can have a concentric outer conduit that can bemade of stainless steel and may be water-jacketed for cooling. The outerwall of the high temperature zone can be water-jacketed for cooling withthe inner wall into which the RF induction coils 28 are embedded, seeFIG. 2D, made of a ceramic material resistant to high temperatures. Aquartz sleeve separator 15 can be mounted above the induction coils ofthe high temperature zone for conveying the plasma-forming gas into thehigh temperature zone. A wall which forms the reactant contacting region14 and at least a portion of the sleeve 19 can be made of aboron-nitride material and can be mounted directly onto a typical plasmajet assembly.

In one embodiment, the invention herein can be construed as excludingany element or process step that does not materially affect the basicand novel characteristics of the composition or process. Additionally,the invention can be construed as excluding any element or process stepnot specified herein as being part of the invention.

EXAMPLES Test Procedures Referenced in the Examples Surface Area

The BET specific surface area of a sample made according to the Examplesis defined as the surface area of one gram of particles. It is definedby the formula:S=6/(Dia)(Den)wherein

-   S is the specific surface area in square meters per gram,-   Dia is the average particle diameter in meters; and-   Den is the density of the particles in grams per cubic meters.

The surface area can be determined by gas absorption (such as N₂) or bydetermining the average particle size by use of an electron microscopeand then using such particle size to calculate the surface area by useof the above formula. Additional information regarding determining thespecific surface area is set forth in T. P. Patton Paint Flow andPigment Dispersion, 1979, John Wiley and Son, Inc. and ASTM MethodC1274.

UPA Particle Size Distribution

The particle size distribution of the particles formed in the Examples,and shown in Table 1, were measured using the ultrafine particleanalyzer dynamic light scattering technique. The MICROTRAC ULTRAFINEPARTICLE ANALYZER (UPA) (a trademark of Leeds and Northrup, North Wales,Pa.) uses the principle of dynamic light scattering to measure theparticle size distribution of particles in liquid suspension. Themeasured size range is 0.003 μm to 6 μm (3 nm to 6000 nm). The dryparticle sample needs to be prepared into a liquid dispersion to carryout the measurement. An example procedure is as follow:

(1) Weigh out 0.08 g dry particle.

(2) Add 79.92 g 0.1% tetrasodium pyrophosphate (TSPP) solution in waterto make a 0.1 wt. % suspension of particles.

As described in the following Examples, the gas phase process andoperating conditions of the present invention were employed in a pilotscale plasma reactor operating at a rate of 3 g/min of titanium dioxidein which the titanium dioxide nanoparticles produced are considered toprovide design data for large scale production.

Example 1

Oxygen gas flowing at 300 standard liters per minute (slpm) and atemperature of 20° C. was introduced to a sleeve 19 of the flow reactor5 through the oxidizing agent inlet 18 upstream of the high temperaturezone. Argon gas flowing at 30 slpm, temperature of 20° C., wasintroduced to a central channel via the plasma-forming gas inlet 17 toform the hot plasma. Local plasma temperatures exceed 10,000° C. In thehigh temperature zone, the high temperature plasma mixed with the oxygento form a hot gas mixture that flowed downwards through the hightemperature zone 12 towards the reaction contacting region. Thetemperature of the oxygen and argon gas mixture in the reaction zone was1000 to 3000° C. The argon and oxygen gases had a near laminar flow.

TiCl₄ vapor flowing at 10 gram/min at a temperature of 140° C. mixedwith argon gas flowing at 10 slpm was introduced into the center of theflow reactor 5 via a ⅛-inch ID inner tube 34. Argon gas flowing at 27slpm was introduced into the outer tube 36. The TiCl₄ vapor and argongas outlets of the metal reactant feed conduit 22 located adjacent tothe reaction contacting region introduced the TiCl₄ to the hot mixtureof oxygen and argon flowing from the high temperature zone. In thereaction zone 16 TiCl₄ reacted with oxygen to form TiO₂ particles andCl₂ gas. The reaction zone extended approximately six inches (152.4 mm)downstream of the TiCl₄ and argon gas outlets and the inside diameter ofthe reactor in the reaction zone expanded from 30 to 80 mm.

An extender pipe 40 comprising a water jacket 41 for circulating coolingwater about the flow reactor down stream of the high temperature zonewas mounted to the bottom of the plasma jet assembly. The water flowedinto the water jacket 41 at a temperature of 20° C.

Oxygen gas was introduced below the high temperature zone 12 at twolocations. The upper location was above the outlet of the metal reactantfeed conduit 24 and just below the high temperature zone 12. The lowerlocation was below the reactant contacting region 14.

In the upper location the O₂ was introduced temperature 20° C. throughthe upper cooling fluid inlet 32 at 440 slpm and flowed co-currenttogether with the reactants into the reaction zone 16.

In the lower location the cool O₂ temperature 20° C. was introducedthrough lower inlet 39 at 440 slpm and flowed co-current together withthe reactants in the reaction zone 16.

The cooled TiO₂ particles were captured in a filter and collected afterthe first hour and after the second hour of a two-hour period ofcontinuous operation. The properties of the resulting TiO₂ powder arelisted in Table 1. As shown in Table 1, the coarse tail (weight percentparticles greater than 104 nm) was less than 5 wt. % for both samplesand the mean particle size (d₅₀) was 44 and 41 nm, respectively. TiCl₄conversion (yield) was 56 and 58%.

No scale was detected. An acceptable light dust was observed on theinterior of the reactor.

TABLE 1 powder average surface particle size data by dynamic lightscattering powder area by particle size distribution by weight innanometers collec- BET wt wt wt tion (m²/g) d₁₀ d₅₀ d₉₀ % >52 % >104% >208 Hour 1 101.2 30 44 79 34.0 4.6 1.7 Hour 2 114.1 27 41 76 28.6 4.21.4

The description of illustrative and preferred embodiments of the presentinvention is not intended to limit the scope of the invention. Variousmodifications, alternative constructions and equivalents may be employedwithout departing from the true spirit and scope of the appended claims.

1. A process for preparing metal oxide particles which are substantiallyfree of coarse tail from an oxidizing agent and a metal reactant in aflow reactor, comprising: (a) directing a flow of the metal reactantinto a contacting region of the flow reactor; (b) passing a flow ofoxidizing agent through a high temperature zone of the flow reactor toform a flow of hot oxidizing agent and directing the flow of the hotoxidizing agent onto the contacting region of the flow reactor at a flowcondition sufficient to form a reaction stream comprising a flow of hotoxidizing agent, a flow of metal reactant and a diffusive flow of thehot oxidizing agent and the metal reactant, the temperature of the hotoxidizing agent being at least sufficient to initiate oxidation of themetal reactant in the diffusive flow; (c) passing the reaction streaminto a reaction zone of the flow reactor at subsonic velocity, whilesimultaneously introducing a cooling fluid substantially coaxially withthe reaction stream to form a fluid curtain which is in contact with andsurrounds the reaction stream under conditions such that the coolingfluid intermingles with the reaction stream by diffusion; (d)maintaining the fluid curtain while the metal oxide particles formwithin the diffusive flow of the reaction stream and until at least amajor portion of the reaction stream is cooled to a temperature belowthe temperature at which metal oxide particles coalesce; and (e)separating the metal oxide particles from the reaction stream.
 2. Theprocess of claim 1 in which the high temperature zone comprises aplasma.
 3. The process of claim 1 in which the metal reactant istitanium tetrachloride and the oxidizing agent is oxygen.
 4. The processof claim 1 in which the flow velocities within the flow reactor aresubsonic.
 5. The process of claim 1 in which the Reynolds Number of thehot oxidizing agent ranges from about 2,000 to about 4,000.
 6. Theprocess of claim 1 in which the metal reactant is a vapor or a liquid.7. The process of claim 1 further comprising introducing a supplementalfluid to the reaction zone.
 8. The process of claim 7 in which thesupplemental fluid is a coolant, adjuvant or dopant.
 9. The process ofclaim 8 in which the coolant is introduced to the central portion of thereaction zone.
 10. The process of claim 1 in which the cross-sectionalarea of the diffusive flow of the reaction stream gradually increasesthrough increasing diffusion of the flows of hot oxidizing agent andmetal reactant within the reaction zone.
 11. The process of claim 1 inwhich the conversion of the metal reactant is no greater than about 90%.12. The process of claim 1 in which the metal oxide particles separatedfrom the reaction stream contain less than about 10% by weight of metaloxide particles greater than about 100 nm in diameter.
 13. The processof claim 1 in which the metal oxide particles separated from thereaction stream contain less than 5% by weight of metal oxide particlesgreater than about 100 nm in diameter.
 14. The process of claim 1 inwhich the metal oxide particles separated from the reaction streamcontain less than 2% by weight of metal oxide particles greater thanabout 100 nm in diameter.
 15. A process for preparing metal oxideparticles which are substantially free of coarse tail from an oxidizingagent and a metal reactant in a flow reactor having in order a hightemperature zone, an intermediate zone, a contacting region and areaction zone, comprising: (a) directing a flow of the metal reactantinto a central portion of the contacting region; (b) passing a flow ofoxidizing agent through the high temperature zone of the flow reactor toform a flow of hot oxidizing agent and directing the flow of the hotoxidizing agent into the intermediate zone for establishing a laminar ornear laminar flow of the hot oxidizing agent and passing the laminar ornear laminar flow of hot oxidizing agent into the contacting region ofthe flow reactor to form a reaction stream comprising a diffusive flowof hot oxidizing agent and the metal reactant, the temperature of thehot oxidizing agent being at least sufficient to initiate oxidation ofthe metal reactant in the diffusive flow; (c) passing the reactionstream into a reaction zone of the flow reactor at subsonic velocity,while simultaneously introducing a laminar or near laminar flow of acooling fluid substantially coaxially with the reaction stream to form afluid curtain which is in contact with and surrounds the reaction streamunder conditions such that the cooling fluid intermingles with thereaction stream by diffusion; (d) maintaining the fluid curtain whilethe metal oxide particles form within the diffusive flow of the reactionstream and until at least a major portion of the reaction stream iscooled to a temperature below the temperature at which metal oxideparticles coalesce and when the metal oxide conversion is no greaterthan 90%; and (e) separating metal oxide particles from the reactionstream, less than 10% by weight of the metal oxide particles beinggreater than about 100 nm in diameter.
 16. The process of claim 15 inwhich the high temperature zone comprises a plasma.
 17. The process ofclaim 15 in which the metal reactant is titanium tetrachloride and theoxidizing agent is oxygen.
 18. The process of claim 15 furthercomprising introducing a supplemental fluid to the reaction zone. 19.The process of claim 18 in which the supplemental fluid is a coolant,adjuvant or dopant.
 20. The process of claim 19 in which the coolant isintroduced to the central portion of the reaction zone.
 21. The processof claim 15 in which the flow velocities within the flow reactor aresubsonic.
 22. The process of claim 15 in which the cross-sectional areaof the diffusive flow of the reaction stream gradually increases throughincreasing diffusion of the flows of hot oxidizing agent and metalreactant within the reaction zone.
 23. The process of claim 15 in whichthe conversion of the metal reactant is no greater than about 85%. 24.The process of claim 15 in which the conversion of the metal reactant isno greater than about 80%.
 25. The process of claim 15 in which the flowof the metal reactant is directed as a jet into the central portion ofthe contacting region.
 26. The process of claim 15 in which the flow ofhot oxidizing agent is near laminar.
 27. The process of claim 15 inwhich the flow of hot oxidizing agent is laminar.
 28. The process ofclaim 15 in which the flow of cooling fluid is near laminar.
 29. Theprocess of claim 15 in which the flow of cooling fluid is laminar. 30.The process of claim 1 in which the cooling fluid is an upper coolingfluid.
 31. The process of claim 30 further comprising introducing alower cooling fluid downstream from the step of introducing the uppercooling fluid so that the lower cooling fluid flows co-current with thereaction stream.
 32. The process of claim 15 in which the cooling fluidis an upper cooling fluid.
 33. The process of claim 32 furthercomprising introducing a lower cooling fluid downstream from the step ofintroducing the upper cooling fluid so that the lower cooling fluidflows co-current with the reaction stream.